This invention relates as indicated to stabilized olefin polymers. It relates particularly to stabilized polypropylene. Still more particularly, it relates to the use of certain phenolic esters as thermal stabilizers for polypropylene. The phenolic esters are derived by preparing monoacetals of pentaerythritol and then esterifying these monoacetals with either a carboxylic acid chloride or a dichlorophosphite. The phenolic group of the ester product resides in the aldehyde from which the intermediate monoacetal is obtained.
Generally, olefin polymer compositions are vulnerable to deterioration of physical and chemical properties during manufacture, storage, processing and use. To overcome such deterioration, or at least to inhibit it, there have been developed additive systems which act to stabilize these polymers with respect to physical and chemical degradation caused by exposure to ordinary environmental conditions. All of these additive systems, however, while effective for their intended purpose, are characterized by one or more shortcomings.
Olefin polymers are especially susceptible to oxidative degradation. The relatively high temperatures required for their customary processing procedures such as roll milling, injection molding, extrusion and the like, invariably promote oxidation because these processes are carried out under ordinary atmospheric conditions, i.e., they are exposed to the oxygen of the atmosphere.
The significance of polymer oxidation lies in the adverse effect it has on the rheology, morphology, color, clarity, gloss and other physical properties. Impact strength may be lost; the surface may become cracked or crazed. Even a darkening of the color may provide a sufficient aesthetic disadvantage as to render the olefin polymer composition unsuitable for its intended use.
U.S. Pat. No. 3,948,946 (Hofer et al.) shows acetals of hydroxybenzaldehydes. The acetals are the reaction products of 2,2-dimethyl-1,3-propanediol, pentaerythritol, ethylene glycol, 1,2-ethanedithiol, toluene-3,4-dithiol, etc. That is, the alcohol precursor is polyhydric. The reaction of pentaerythritol, however, is carried out to completion, i.e., all of the aliphatic hydroxy groups are acetalized. The acetals are said to be effective stabilizers for organic materials.
U.S. Pat. No. 4,013,619 (Schmidt) shows acetals of certain hydroxyphenylacetaldehydes and hydroxyphenylpropionaldehydes, in some instances (see Columns 16 and 17) with pentaerythritol residues. The acetals are either monoacetals or diacetals, but the monoacetals do not contain unreacted aliphatic hydroxy groups. The acetals are said to be effective heat stabilizers in synthetic resin compositions.
U.S. Pat. No. 4,151,211 (Hechenbleikner et al.) shows acetals of 4-hydroxyphenylpropionaldehydes and such hydroxy or mercapto compounds as pentaerythritol, dodecyl mercaptan ad various other acetalizing reactants, as well as their use in stabilizing polypropylene. None of the acetals, however, contain unreacted aliphatic hydroxy groups.
French Pat. No. 2,310,558 shows certain diacetals of pentaerythritol and 3,5-ditertiarybutyl-4-hydroxypropionaldehyde and 3,5-ditertiarybutylbenzaldehyde.
The invention of this application is a phosphite ester of a pentaerythritol monoacetal having the structure ##STR1## where R is alkyl, cycloalkyl or aralkyl having 3-10 carbon atoms, R.sup.1 is alkyl or 1-6 carbon atoms, R.sup.2 is lower alkyl or hydrogen, X is an organic radical, and n is 0-3.
The invention also includes the process of preparing such esters comprising reacting a monoacetal of pentaerythritol having the structure ##STR2## where R is alkyl, cycloalkyl or aralkyl having 3-10 carbon atoms, R.sup.1 is alkyl of 1-6 carbon atoms, R.sup.2 is lower alkyl or hydrogen, and n is 0-3, with an ester-forming compound having the structure Cl.sub.2 P-O-X, wherein X is an organic radical. The term "lower alkyl" denotes an alkyl group having 1-4 carbon atoms.
Illustrative species of R include methyl, ethyl, isopropyl, tertiarybutyl, tertiaryamyl, 2,2'-dimethylbutyl, cyclopentyl, cyclohexyl, 2-methylcyclohexyl, benzyl and phenylethyl; illustrative species of R.sup.1 include methyl, ethyl, isopropyl, tertiarybutyl, tertiaryamyl and 2,2'-dimethylbutyl; illustrative species of R.sup.2 include methyl, ethyl, n-propyl, isobutyl and hydrogen. Preferably, at least one of R and R.sup.1 is a bulky group, e.g., tertiarybutyl, phenylethyl or cyclohexyl.
The organic radical X is an aromatic radical, i.e., it contains a benzene ring. It may be a substituted aryl group, i.e., an alkylphenyl group (where the alkyl has 1-6 carbons) such as 4-tertiarybutylphenyl, 2,4-ditertiarybutylphenyl or 2,6-diisopropylphenyl; or a hydroxyphenyl group such as 4-hydroxy-2-methyl-3-tertiarybutylphenyl, 4-hydroxy-2,3-ditertiarybutylphenyl or 4-hydroxy-2-tertiarybutyl-5-n-octylphenyl. The aromatic radical may be one which is attached directly to the oxygen of the P-O group, i.e., through a benzenoid carbon atom, or it may be attached through an aliphatic carbon atom, e.g., benzyl, 2-phenylethyl, 2-(4-hydroxyphenyl)ethyl and 2-(4-hydroxy-3,5-ditertiarybutylphenyl)ethyl.
In general, X is phenyl, alkylphenyl, or (hydroxyphenyl)alkyl, where the alkyl group(s) in each case have 1-6 carbon atoms.
The process of the invention involves reacting the above pentaerythritol acetal with the acid chloride under such conditions as to cause the evolution of hydrogen chloride. The reaction is slightly exothermic and it is accordingly advisable to employ external cooling to control the reaction. Stoichiometric quantities of the reactants should be employed for best results, i.e, two mols of carboxylic acid chloride per mol of pentaerythritol monoacetal to form carboxylic esters, or one mol of the dichlorophosphite per mol of pentaerythritol monoacetal to form the phosphite esters of this invention.
A hydrogen chloride acceptor is employed, usually a tertiary aliphatic amine such as triethylamine or tri-n-butyl amine, i.e., one having 3-12 carbon atoms, and the reaction is best carried out in a solvent. Typical solvents include toluene, dioxane, benzene, and the like. Any inert solvent is suitable. The temperature of the reaction ordinarily is within the range of from about 10.degree. C. to about 100.degree. C.
The reactants, solvent and hydrogen-chloride acceptor are mixed, stirred until reaction is complete and the desired solid product separated. If a pure product is desired, crystallization from a hot aliphatic hydrocarbon (such as hexane) usually will serve that purpose.
The process is illustrated by the following examples.